This invention generally relates to video signal processing apparatus, and more particularly to a signal processing system for stabilizing a television picture reproduced from video produced by an unsteadily held camera.
Commonly assigned application Ser. No. 319,845 filed Nov. 10, 1981 by Richard R. Green, Henry W. Mahler and the present applicant, entitled "Television Picture Stabilizing System", the disclosure of which is hereby incorporated herein by reference, describes a signal processing system for detecting and measuring picture unsteadiness in recorded or live video, and automatically eliminating such unsteadiness without destroying the effects of intentional panning and tilting of the camera. A video signal from an unsteadily held camera is passed through a variable delay device having a controllable delay time which is adapted to introduce an off-setting delay equivalent in time but in a direction opposite to the motion that would otherwise be observed in the television picture as a consequence of camera motion. This is accomplished by selecting camera motion references within the camera field of view consisting of one generally horizontally disposed edge and one generally vertically disposed edge of an object which would normally be stationary, such as a door, a window, or a piece of furniture. The stabilizing system "locks" onto these edges, continually measuring their motion from frame to frame, and utilizing suitable delay devices makes the necessary vertical and horizontal corrections to the overall picture to substantially eliminate such motion. The system is able to separately track the moving edges while recognizing the important difference between random unsteadiness and planned camera movement.
Although the system of the copending application acceptably performs the function outlined above, the fact that each or both of the selected horizontal and vertical reference edges may in some situations move both horizontally and vertically makes it inconvenient at best and often difficult for the operator to keep the system "locked on" to the selected reference edges. The nature of the problem will be best understood by consideration of a specific example. Considering first a situation where simultaneous horizontal and vertical motion does not present a problem, imagine a scene in which a telephone pole extends beyond the top and bottom of the picture, an ideal reference for horizontal stabilization. In the system described in the copending application a joystick-controlled cursor is positioned over a portion of the pole to initialize the horizontal stabilization portion of the system, and once initialized the horizontal stabilizer "looks" for this reference edge once every field at a vertical position determined by the joystick and at a horizontal position centered at a point computed by adding its previous motion to its previous horizontal position. Thus, the operator is required to keep the vertical position of the joystick-controlled cursor at a position somewhere between the top and bottom of the vertical reference edge. If a telephone pole is available as the vertical reference there is no problem since the picture could move a large distance vertically, either up or down, before the cursor would have to be moved. However, most real world scenes do not present such convenient objects for use as references. Suppose, for example, that instead of a telephone pole the only object in the scene that might serve as a vertically disposed reference edge is a stop sign which may be only one-tenth or one-twentieth of the total picture height. As in the case of the telephone pole, the operator can position the cursor on the sign post and use it for horizontal stabilizaton. However, if the picture moves up or down by any substantial amount, the operator is presented with the difficult task of maintaining the cursor between the top and bottom extremities of the signpost. The shorter the reference object is relative to the overall height of the picture, the more precise is the requirement for vertical positioning of the cursor on the vertical reference edge. To simplify the operator's task in situations of the type described, and in situations where the scene is complex and offers only short reference edges, the system desirably should be capable of automatically moving the horizontal reference selection cursor vertically and in the same direction as the picture is moving, and of also moving the vertical reference selection cursor horizontally in the same direction and by the amount the picture is moving horizontally.